Do I understand you correctly that by "relvalue" you mean "any possible
value from any possible domain that can occur within a relation" ? You
have already pointed out that, acoording to Codd, each defined relation
establishes a new domain of its own. I take it that the values of such
a domain are indeed "relation values". It would then follow, if I
understand you correctly, that all relation values are "relvalues" but
not all "relvalues" are relation values.